I'm returning to boarding after a break of a few years (no good reason for that!) and bought a 133 Futura and 105 Kombat the end of last summer. Love the boards, and I'd appreciate your thoughts on sail sizes...

Currently using a 6.5 and a 7.5 on the Futura 133, and looking to get a bigger sail. I'm guessing 8.5 - 9.0 would be about right. Any thoughts on that, and what the right fin would be for the big sail?

If you alraedy use a 7.5, then go all the way to a 9m sail, 8.5 a bit too close. F133 will take a 9, it's 77 wide.
Not sure about the fins, if you got a 48 with your F133, try your new big sail with it first. Then buy bigger and/or more powerfull fin if you need it.